1. Field of the Invention
The present invention relates to a coating apparatus to apply treatment liquid to a recording medium and an image forming system including the coating apparatus.
2. Description of the Related Art
Image recording of an inkjet method is becoming increasingly popular these days because of its advantage that colorization can be easily realized, in addition to its advantages of low noise and low running cost. But, when an image is formed on a recording medium which is a non-manufacturer-specified paper, problems related to initial image quality such as image blur, image concentration change, color tone change, image show-through, etc., occur. In addition, problems related to robustness of images such as water resistance, weather resistance, etc., occur.
In order to solve these problems, in one image forming apparatus proposed in JP-H07-156538-A, treatment liquid having a function to agglomerate the ink is applied to a recording medium just before ink droplets are ejected onto the recording medium (paper).
The above-mentioned Patent document disclose a method to apply the pretreatment liquid on the entirety of the sheet, using rollers.
FIG. 8 shows a schematic diagram illustrating a configuration in which coating material (treatment liquid) is applied to a sheet of recording media using a roller, according to the related art. In FIG. 8, a sheet of recording media W is wound around a platen roller 97 rotated by a motor, using a paper pressing member 98. Coating material 90, contained in a coating unit 96, is drawn up by an agitation-supply roller 91 and then is applied to a roller surface of a coating roller 94 by conveyance thin-coating rollers 92 and 93 to form a thin coating on the surface of the coating roller 94.
As the coating roller 94 is rotated with the rotation of the platen roller 97 while the coating roller 94 presses against the recording medium W that is wound around the platen roller 97, the coating roller 94 applies the coating material 90 to a surface of the recording medium W. Along with these processes, an ink ejecting head 95 applies ink to the surface of the recording medium W that is coated with the coating material 90. This configuration is proposed in, for example JP-2012-053332-A.
The method of applying the treatment liquid that improves the image quality to an image area of the recording medium in advance using the roller enables the applied treatment liquid to have higher viscosity than the method using a sprayer (e.g., a spraying head). Therefore, the method using the roller is superior to the method using the sprayer in forming a thinner coating and lessening the image blur.
However, in the above-described coating device that uses the roller to apply the treatment liquid, the roller is readily abraded at the portion thereof that comes in contact with the edges of the sheet. When the sheet width is changed, due to lack of the liquid in abraded portions of the surface of the coating roller, white dots may appear in the image formed on the recording medium, and due to excess liquid arising from the accumulated liquid in the abraded portions, unevenness in the image density may occur.